One of the main but limited radio resources in networks for mobile telecommunications of code division multiple access in particular of wideband W-CDMA type, is channelisation code. The problem of how different channelisation codes are allocated to different call-connections is dealt with using a channelisation code allocation strategy. The objective of the channelisation code allocation strategy is to enable transmissions to as many users as possible with minimum complexity, as that is the most efficient way to utilise the codes.
The channelisation codes to be used in W-CDMA are orthogonal variable spreading factor OVSF codes. By using orthogonal variable spreading factor OVSF codes, variable data rates can be supported.
The orthogonal variable spreading factor OVSF codes are presented as a code tree diagram in FIG. 2. Each layer of the tree has a different spreading factor SF and for each spreading factor SF value, there is an equal number of possible codes, i.e. if spreading factor SF=8, there are 8 codes available for that spreading factor SF. Each code in the code tree is denoted in the form CSF,k, where k is the code number.
From the orthogonal variable spreading factor OVSF code tree diagram (FIG. 2), it will be seen that codes on different layers of the tree have different lengths. During spreading, each information bit is represented by an entire codeword, so, in the known approaches, longer codes (high spreading factor SF) are used for transmissions to lower bit rate users. It follows that a high-rate user is allocated a low-spreading factor SF code, and a low-rate user is allocated a high-spreading factor SF code. This known code allocation strategy thus focuses on allocation of channelisation codes dependent upon expected data rates. This allows for fairness to all the subscribers or radio bearers associated with a subscriber. However, this is not always desirable for example where subscribers are viewed differently by operator depending on the tariff they subscribed to. In addition, some radio bearers associated with a user terminal UE are much more sensitive to delay than the others.
On a related matter, referring back to FIG. 2, a parent code is a code that generates higher spreading factor SF codes in the next layer of the code tree. For example, code C2,0 is the parent code of codes C4,0 and C8,0. A child code has a higher spreading factor SF code compared with its parent code, i.e. code C8,0 is the child code of codes C4,0 and C2,0. The codes contained in each layer of the code tree are mutually orthogonal. In addition, any two codes from different layers of the code tree are also orthogonal, except when one code is the parent or child of the other code. Therefore, in order to preserve orthogonality, the use of the orthogonal variable spreading factor OVSF codes has two restrictions:
1) If a code e.g. C8,0 is in use, this means that its parent codes, e.g. C4,0 and C2,0, cannot be used.
2) Likewise, if a code e.g. C2,0 is in use, the child codes, e.g. C4,0, C4,1, C8,0, C8,1, C8,2, and C8,3, cannot be used. This gives rise to a problem of only a limited number of codes being available. Particularly in W-CDMA, code shortage is quite severe in the downlink channel
The known approach to this problem of the shortage of available codes is to introduce further downlink scrambling codes. Each downlink scrambling code provides a fresh code tree i.e. a fresh set of channelisation codes. The disadvantage of this solution is that the level of interference on the downlink (i.e. from a base station to a user terminal) is increased due to the loss of orthogonality between the code trees of the various scrambling codes.